Electrical circuit elements



Dec. 15, 1964 R, A, H|| Erl-AL 3,160,944

ELECTRICAL ciRCUT ELEMENTS Filed` April 1l, 1960 Inventor.

Ronald A. H111 Alan W. Stirling United States Patent 3,160,944 ELECTRCAL CIRCUT ELEMENTS Ronald Alfred Hiil and Alan William Stirling, London, England, assigner-s to International Standard Electric Corporation, New York, N.Y.

Filed Apr. 11, 1960, Ser. No. 21,430 Claims priority, application Great Britain, Apr. 24, 1959,

3 Claims. (Cl. 29-25.42)

This invention relates to electric circuit elements which include a body containing high dielectric constant ceramic material, and which may be used to form circuit components, such as variators, rectiiers, and capacitors.

The use of high dielectric constant ceramic materials for the manufacture of capacitors is known, and capacitors made with bodies of these materials have a high capacitance per unit area and volume.

Typical compositions used for these bodies contain the titanates of barium, strontium and calcium, to which other additions are made chiefly for the purpose of producing either improved ageing, or greater stability of properties with temperature.

Attempts have been made to increase the capacitance of these elements by decreasing the thickness of the ceramic material between the electrodes, but the results are unsatisfactory because of the brittle nature of the materials. Another method which has been used takes advantage of the fact that these'ceramics can be made conducting by the addition of rare earth oxides.

In making a capacitor employing dielectric ceramic material it is known to make a ceramic body consisting essentially of an alkaline earth metal titanate or titanates to which Vhas been added a small percentage (e.g. 0.5%) of rare earth oxides, specifically lanthanum oxide, by a Y method which includes heating in an oxidising atmosphere to produce the ceramic body, heating in a reducing atmosphere to make the body semi-conducting and then applying silver electrodes by firing in an oxidising atmosphere. The latter step is stated to produce a thin insulating layer beneath each electrode.

The resulting capacitors have comparatively high capacitance per unit area, but are unfortunately only useful for voltages up to 4 volts, because of the extremely rapid increase in leakage current above this voltage.

By using a rare earth titanate as an additive to a body of high dielectric constant ceramic material in place of lanthanum oxide the present invention makes it possible to produce immediately beneath an electrode a dielectric layer of very high resistivity which has a higher breakdown strength than can be obtained with ceramic bodies in which lanthanum has been incorporated as an oxide.

A particular method of making a capacitor in accordance with the present invention includes the following steps.

(l) Making a mixture of barium titanate, or a mixture of barium and strontium titanates, with a certain proportion of lanthanum or other Arare earth titanate. Known ceramic techniques, such as incorporating in the mixture certain proportions of binder and lubricants, are used.

(2) Pressing this material into suitable shapes, such as discs.

(3) Firing in an oxidising atmosphere at a temperature between 1300 C. md 1500 C. for several hours to form ceramic bodies.

(4) Reducing these ceramic bodies in hydrogen or other suitable reducing atmosphere for .about one hour at 900 C. v

(5) Applying a silver composition to the faces of the discs and tiring in an oxidising atmosphere at a comparatively low temperature e.g. 500 C.

ICC

(6) subjecting the silvered discs to re-oxidation at a temperature up to the melting point of silver for a suitable period.

As mentioned above, it is a known practice to produce conducting ceramic bodies by the use of a small proportion of lanthanum or other rare earth oxide. This proportion does not usually exceed 1 mole percent. An essential step in the process of the present invention is to use a mole proportion, for example 5%, of lanthanum or other rare earth oxide in stoichiometric proportion with titanium dioxide. The compound in the case of lanthanum is LA2O3.3TiO2 or expressed in another way La2/3Ti03. This compound is known to have the same perovskite structure as barium or strontium titanate but with one third of the lattice points normally occupied by lanthanum atoms vacant. It is sufficient to incorporate the appropriate quantities of lanthanum oxide La203 and titanium oxide TiOZ in the barium or barium/ strontium titanate material, but it is preferred to make the compound previously by mixing the constituent oxides intimately and tiring at 1250 C. for one hour. The resulting white compound is then added to the main titanate composition together with the binder and lubricant for pressing.

The ceramic body produced at step (3) above has a resistivity of between 101 and 1012 ohm-centimetre, and it can be regarded as a typical insulating ceramic body. After reduction at step (4) the resistivity may be as 10W as one ohm-centimetre. The unit provided by step (5) with the silver electrodes in place and with an area (each face) of one square centimetre has a capacitance in the region 1 to 5 microfarads. At this stage of manufacture the unit has a high conductivity and is not suitable for use as a capacitor. It is then oxidised by heating it in an oxidising atmosphere for a period of time which may be varied in accordance with the capacitance and working voltage which is required. Oxidation takes place because of the high solubility of oxygen in silver up to the melting point of the metal, which permits easy diffusion of oxygen through it. This step results in the formation of a highly insulating layer under each of the silver electrodes, the thickness of which, knowing the capacitance of the unit and the known dielectric constant of the composite ceramic, is estimated to be about 25 microns (l mil). are connected with one another through the body of the conducting part of the ceramic, the eifective capacitance of the unit is half that of each capacitor so formed.

The following approximate values of capacitance are reached for the following periods of oxidation in air at 900 C. f

0.5,nf. per square centimetre-l/z hour 0.1-0.3/tf. per square centimetre-l-Z hours 0.05-0.l,af. per square centimetro-2 3 hours A section through a capacitor unit made in accordance with the present invention and a diagram showing the resistivity of the unit across its section as an ordinate will now be described with reference to FIGS. 1 and 2 of the accompanying drawing respectively.

Referring to the drawings, 1 represents the conducting part of the ceramic body, 2 and 2a the 'dielectric layers, 3 and 3a the silver electrodes, and 4 and 4a the leads forming the terminals of the capacitor. These leads are of silver wire suitably attached to the electrodes by means of blobs of silver composition 5 and 5a similar to that used to form the electrodes. The unit may be protected from moisture by varnishing, by encapsulation, or by other known means, but tests have shown that it is little yaffected by normal atmospheric conditions over periods of several months even without this protection.

Curve 6 shows the resistivity of the reduced body to As the two dielectric layers so formed be relatively high in the middle and low at the faces. Curve 7 shows that afteroxidation through the silver electrodes the surface layers attain a high resistivity while the interior remains almost unalfeoted. The result off this is a sharp division between the dielectric layers `and Athe interior.

wCR, where w=angular frequency, R-:series resistance, and C=capacitance, remains at a low figure. The power factor of units made by the method described is about tol0%. .v l

Capacitors having characteristics within thefollowing ranges have been made vby the process described. 'The capacitance values are also given in terms of apparent dielectric constant(K) for a unitof one square centimetre per face and a thicknes of one millimetre.

Insulation Resistance in M egohms Capacitance, pf.

Apparent K At the same time, the interior remains of sui` ciently low resistivity so that power factor, given byA 5o, ooo-10o, 000 o, 00H00, 000

70-300 at 30 Volts.

Tests have also been made lon compositions contain- Y ing varying proportions of lanthanum titanate and also tests on other temperatures of hydrogen reduction'. We have found that a portion of lanthanum titanate above Ya certain minimum, and also reduction above a certain temperature (about 800 C.), is necessary Kto obtain the most highly insulating layers. Contrary to expectation, lightly reduced bodies are not the most easily re-oxidised.

It is also the case that there is a greater degree of re- Y duction in the outer layers of the hydrogen reduced body of step (4) than inthe interior. By' oxidation-.therefore Vin step (6) the layer ofvceramic just under the silverv electrodes increases most in resistance. This is shown in curve7 of FIG. 2.

Further tests have establishedwthat` diifusionof silverV which takes place during step (6) also helps the oxidation process. tion ofthe compound (AgLa) TiO3. Monovalent silver ion-sAg-loppose the influence of the trivalent lanthanurn ions La3+ and help the development of highinsulat-ion resistance in the. dielectric layers. f

Tests have also been made on single sided units in into the surface and re-applying a silver layer fired on at v 550 C. in an oxidising'atmosphere.

A suitable composition for the manufacture of capacitors according yto the described method is: Y

250-300 at 50-70 Volts.

The materials are ball-milled for 2 hours, after which the slurry is removed to a shallow basin and the water removed by evaporation in an oven at 100-110" C. The mixture remaining is then ready for pressing and tiring. The density of the red discs should be 5.6-5.8 gms./cc.

"According vto'our preferred process, the proportion of lanthanum titanate should be from 4-8 mol percent of fthe amount of bariurnr(or barium strontium) mixture .Y used.f Below this proportion the insulating properties of l()V the oxidised layersv are not so good. Above this proportion the power factor of the capacitors deteriorates. Although in the particular method described the ceramic Y body is reduced at 900 C., this step may take place at a temperature in the range 800 to 1200o C. in any well known manner. Similarly the tiringY of the electrodes in the particular method described is at 500 C., but this vstepmay be'performed at any suitable temperature up ,p to 900 C. `The reoxidised layer under the electrode may vbe produced whenl the electrode is .tired on or as aA separate step. v y vWhile the principles of the invention have been described above in connection with specific embodiments,

Y and particular modifications thereof, it is to be clearly 1 understood that this description is made only by Way of example and not as a limitation on the scope of the invention. What we claim is: l

1. A method of manufacture .of a circuit element which comprises mixing barium titanate with lanthanum Barium titanate gms-.. Lanthanum titanate grns 5 Methyle cellulose gms 1.5 Stearic` acid gms 0.75

..-f*fr-wwfffe=rv -'fe--F'r-U-fmlvf The reason for this is probablythe forma- ,49'

titanate and a'binder shaping the mixture thus formed into a body having a predetermined shape, firing the body in anV oxidisingatmosphere to produce an insulating ceramic body, iii-ing the insulating ceramic body. in a reducing atmosphere `to produce a conducting ceramic body, tiring a metal electrode on to the conducting ceramic body in an oxidising atmosphere and producing an oxi- Vdised layer beneath the electrode.

. 2. A method of manufacture of an electrical circuit element comprising mixing ceramic dielectric materials with -a rare earth titanate in the proportion Yof between tand 8 mol percent of the said materials, pressing the mixture thus formedinto a body of predetermined shape, firing the body in an oxidising atmosphere at a temperature between 1300 C. and 1500 C. torproduce an insulating ceramic body, tiring the resultant body in a` reducing atmosphere at a temperature between 800 C. and 1200 C. to produce' a conducting ceramic body, firing a silver electrode on to the conducting ceramic body in an oxidising atmosphere at a temperature not in excess of 900 C. and producing an oxidised layer beneath the electrode.

3. A method of manufacture, as claimed in claim 2, in which the said ceramic dielectric materials include barium and strontium titanates.

Y References `Cited in the file of this patent p UNITED STATES PATENTS 2,520,376 Roup Aug. 29, 1950 2,633,543 1 Howatt Mar. 31, 1953 2,712,172 Bayha July 5, 1955 2,721,966 Jenkins etal. Oct. 25, 1955 2,990,606 Napolin July 4, 1961 

1. A METHOD OF MANUFACTURE OF A CIRCUIT ELEMENT WHICH COMPRISES MIXING BARIUM TITANATE WITH LANTHANUM TITANATE AND A BINDER SHAPING THE MIXTURE THUS FORMED INTO A BODY HAVING A PREDETERMINED SHAPE, FIRING THE BODY IN AN OXIDISING ATMOSPHERE TO PRODUCE AN INSULTING CERAMIC BODY, FIRING THE INSULATING CERAMIC BODY IN A REDUCING ATMOSPHERE TO PRODUCE A CONDUCTING CERAMIC BODY, FIRING A METAL ELECTRODE ON THE CONDUCTING CERAMIC BODY IN AN OXIDISING ATMOSPHERE AND PRODUCING AN OXIDISED LAYER BENEATH THE ELECTRODE. 